Capillary electrophoresis is a separation method employed in analytical chemistry which utilizes the differences in electrophoretic mobility of the sample substances to be separated. Capillary electrophoresis is used, for example, for separating different biological molecules, such as proteins or peptides. The separation process is performed in a capillary tube which is open on both ends and to which an electric field is applied which causes electrophoretic separation of different sample substances within the tube. The electric field is applied by means of electrodes which are arranged at the ends of the capillary, respectively, and which are connected to a high voltage power supply. The capillary is filled with an electrically conductive electrolyte so that an electric field can build up within the capillary. The two ends of the capillary are immersed in vials containing the electrolyte, respectively.
When sample substances are to be introduced into the capillary for subsequent separation, the vial containing the electrolyte is removed from one end of the capillary, a vial containing the sample is positioned at this place so that the end of the capillary is immersed in the sample liquid. Thereafter, the sample is injected into the capillary by a suitable method, for example by applying a pressure above atmospheric pressure at the end of the capillary where the vial is positioned or a vacuum at the other end of the capillary. When the sample substances have been injected into the capillary, the sample vial is removed and the electrolyte vial is again positioned at this place. Thereafter, high voltage is applied so that electrophoretic separation of the sample substances takes place. At the end of the capillary opposite to the end of sample injection, a detector is arranged for detecting the separated sample substances by a suitable detection method, for example by a light absorption or a fluorescence technique.
A frequently used method for introducing liquids, such as electrolyte, sample liquid and rinsing liquids into the capillary, is to apply a pressure difference between the two ends of the capillary. This pressure difference forces liquid into the capillary. When applying this method, the capillary has to be sealed against the vial in which the liquid to be introduced is contained to prevent escaping of the pressure to the outside.
In the prior art, there are different solutions for designing the mentioned seal between capillary and vial. According to a first solution, liquid is introduced into the capillary by applying a pressure below atmospheric pressure at the outlet end of the capillary where the detector is located. In this case, the electrolyte, vial at the outlet end of the capillary is connected with the capillary by a screw fastening. The screw fastening also performs the sealing function. This design has the disadvantage that it is not possible to exchange the electrolyte vial at the outlet end of the capillary during an analysis sequence since the end of the capillary and the vial are rigidly connected. Furthermore, since the introduction of liquid is performed by application of under-pressure, there is only a maximum pressure difference of 1 bar available. In practice, the usable pressure difference would only be about half of this value because the gas emissions from the liquids would otherwise be too strong.
In a second design solution for the sealing of the capillary against a vial, the capillary is disposed in a cartridge body and the two ends of the capillary protrude vertically downward from the bottom of the cartridge, at two spaced apart locations at the bottom of the cartridge. These two ends are sealed about their periphery with respect to the cartridge where they exit the cartridge. For introducing liquid into the capillary, the corresponding vial is pressed against the bottom of the cartridge, then an over-pressure is applied on the liquid in the vial, forcing the liquid into the capillary. A capillary electrophoresis apparatus using this design is known from EP-A-0 339 780.
With such a known design, a sealing of the periphery of the capillary to the cartridge is achieved by injecting a sealant, preferably a silicon rubber compound, into apertures near the bottom of the cartridge. A design of that type is not satisfactory in all respects. When the capillary in the cartridge is to be replaced, for example when the capillary is broken, a user has to remove the sealant, and then, after insertion of a new capillary, he has to renew the sealing by filling in sealant into the corresponding apertures in the cartridge. This is a time-consuming process and requires that the user has the sealing material (e.g., silicon rubber) at his disposal. On the other hand, if the user wants to avoid this time-consuming process and just takes a new cartridge with a sealed capillary inside, this becomes more expensive since he also has to bear the cost for the cartridge, whereby the cost is still increased by the sealing.